When we had our Starcraft RT10 it was designed with a larger frame to go along with the lifted suspension and shocks. Although it was purpose built it had its problems. The first major off-road trip we took it came back leaning to one side from the leaf springs collapsing. Typical RV manufacturer built it with a load capacity of almost zero.
I really dont think it would take a huge investment to outfit a standard pop up to go off-road. You could always run some extra frame supports to beef up the frame if needed. plus install all the appropriate gear for what YOU need.